Patent GB-1118961-A (GB)
Title: Terrain-following system
Inventor: ANDERSON JAMES ORVILLE
IPC: G01S13/935
Filed: 1965-04-06
Abstract: 1,118,961. Radar. NORTH AMERICAN AVIATION Inc. 6 April, 1965, No. 14660/65. Heading H4D. A terrain following system for an airborne vehicle enables the vehicle to travel a preselected clearance distance above the ground and to avoid obstacles with a similar clearance, the up and down obstacle-avoidance manoeuvres performed by the vehicle being kept within predetermined limits both in respect of the vertical acceleration imposed on the vehicle and also the vertical angle of the vehicle flight path. Referring to Fig. 7, there is illustrated a closed loop airborne vehicle control system according to the invention. There is provided a vehicle controller 40, a controlled vehicle 41, and an altitude sensor 42 in closed loop cooperation as an altitude control system. The controlled vehicle may be an aircraft or other airborne vehicle operated in close proximity to, and above, a terrain obstacle which is to be avoided; and the controller may be an automatic flight controller or autopilot known in the art for controlling a vehicle in response to a control signal. Accordingly, elements 40 and 41 are shown in block form only. Altitude sensor 42 may be a downward looking radar or other known means for measuring the height of a vehicle above terrain, and is therefore shown in block form only. The altitude signal provided by sensor 42 is compared with an altitude reference signal by signal comparison means 43 to provide a control signal indicative of the difference therebetween. Such control signal is employed by the controller 40. However, such control arrangement alone is not wholly adequate for terrain-following control of a high-speed vehicle because adequate forward range terrain profile data is not provided, whereby the vehicle is enabled to successfully maneouvre so as to both follow, and safely avoid, the terrain profile. Accordingly, there is also provided a forward-looking terrain sensor 44 in co-operation with a terrain-profile signal means 45 for providing signals indicative of the terrain profile in a desired direction such as, for example, parallel to the projected flight path ahead of controlled vehicle 41 (and displaced therefrom a perpendicular distance h 0 ). There is further provided adaptive clearance surface signal means 46 responsive to forwardlooking sensor 44 and the flight path angle of the vehicle 41 for generating signals indicative of preselected manoeuvre limits of the controlled vehicle 41. Signal combining means 47 combines the outputs of the terrain profile generators 45 and the adaptive clearance surface signal means 46 to provide a terrainfollowing control signal indicative of the difference between the sensed terrain profile and the synthesized manoeuvre-limited clearance plane. Such control signal is fed to the vehicle controller 40 for automatic control of the controlled vehicle 41, in such a sense as to tend to reduce the magnitude of, or oppose the sense of, the difference signal. Alternatively, such control signal may be fed to a display device or other indicator means, whereby a pilot or human operator may be enabled to control a controlled vehicle manually in order to perform effectively and safely a terrain-following mission. The approach to a terrain obstacle during the vehicle climb, and the initiation of the pull-power manoeuvre in response to the coincidence of the terrain obstacle with the control surface are shown in Figs. 10a, 10b, 10c. Referring to Figs. 10a, 10b, 10c, there are illustrated three successive positions respectively of a vehicle over an ideal climb flight profile. There is provided a terrain profile 54 having a terrain obstacle 55. Curve 71 describes an ideal flight profile comprising a pull-up manoeuvre to a pull-over point 72 and then a pull-over manoeuvre to the top of obstacle 55. Curve 73 represents the pull-up clearance control or plane similar to the pull-up control surface described by points 66, 67, 68, 69 and 70 in Fig. 9. Point 74 in Fig. 10a represents a position of a vehicle along the ideal flight profile 71 and below the pull-over point 72, and curve 75 emanating from position 74 represents a control curve having a radius of curvature R TD  and being tangent to first climb clearance plane 73 at point 76. The position of the tangent point 76 below, and at a shorter range than, the upper extremity of the controlling terrain obstacle 55, indicates that the aircraft is to continue to employ the first climb control surface 73. Fig. 10b illustrates that the point of tangency is coincident with the terrain obstacle, indicating that the position of the aircraft (point 72) is at that point on the flight profile at which the turn-down acceleration manoeuvre is to be initiated (and the turn-down control surfaces 75 employed in lieu of the first climb control surface 73). Fig. 10c illustrates that the point of tangency of curve 75 to curve 73 is above, and at a further range than, terrain obstacle 55, indicating that the position 74<SP>11</SP> of the controlled vehicle is past the point 72 at which the pullover manoeuvre is initiated. Accordingly, the terrain following control system will continue to use the pull-over control surface 75<SP>11</SP> and will continue the turn-down acceleration or pullover manoeuvre. The additional control surface 75 of Fig. 10 is employed in conjunction with the first control surface in climbs. When the vehicle passes over the terrain obstacle, the vehicle is controlled by the sense of the difference between the terrain profile and the control surface. In other words, when all terrain is below the clearance plane (say curve 49a in Fig. 8b), then the sense of the control signal directs the vehicle to change its flight angle (γ) by a negative amount (- #γ), the vehicle controller being " - g " limited and dive-angle limited so as to prevent the vehicle from exceeding predetermined dive manoeuvre limits. These curves of (##sR,γ) are generated in function generator 46 in Fig. 7. The control surface is deemed, therefore, to be comprised of four curves, I, II, III and IV, as shown in Fig. 11, describing the bias signal, ##sR as a function of γ. In Fig. 11, Curve I represents that initial clearance plane shape and position applying in pull-up from a dive to a predicted horizontal flight path (γD<γ<0) and is generated for ranges from zero up to the range (R 1 ) of the predicted horizontal position. Curve II represents that portion applying beyond range R 1  or in a pull-up during a climb (0<+γ<+γc) and is generated up to the range R 2  at which maximum flight path angle γc occurs. Curve III applies either beyond the range R 2  or when the vehicle in a limiting climb (+ γ = γc) or in clearing terrain obstacles within the maximum range or view-angle limits of the sensor. Curve IV applies in a climb (+ γ) for initiating a pull-over manoeuvre at a range R 4  from a " worst " terrain obstacle. In other words, for a vehicle in a dive (-γ), curves I, II, III are generated within the range limits (0-R 1 ), (R 1 -R 2 ) and (R 2 -R 3 ) respectively; while in a climb, curves II, IV and III are generated within the range limits (0 and R 2 ), (0 and R 4 ) and (R 2  and R 3 ) respectively. Accordingly, the control surface signal generator 46 of Fig. 7 is required to generate control surface signals indicative of curves I, II, III and IV of Fig. 11. A generalized block diagram of such structure is shown in Fig. 12. Referring to Fig. 12, there is illustrated a more detailed block diagram of the system of Fig. 7. There is provided a radar system 44 for providing a video gating signal indicative of a radar return signal and a signal indicative of the target angle N relative to the FRL of the controlled vehicle in which such system is installed. In an on-boresight system, such angle may be indicative of a variable antenna angle N (t), or in an off-boresight system might be indicative of the sum of an antenna angle N 0  and a target angle off-boresight #. There is further provided an inertial reference 78 such as a vertical gyro for providing signals indicative of the inertial attitude # of the vehicle FRL, and an angle-of-attack sensor 79 for providing a signal indicative of the angle of the vehicle velocity vector relative of the vehicle FRL. Combining the outputs of sensors 78 and 79 by means of signal combining means 80, a signal is provided which is indicative of γ, the flight path angle of the vehicle relative to the inertial reference. This signal may be fed on a line 84 to a summing amplifier 81. Combining the radar system view angle signal N and the output signal #, by means of a gated summing amplifier 81, provides a resulting view angle signal referenced to the same inertial reference as the vehicle flight path, γ. The summing amplifier 81 is gated by the radar return or receiver signals, to provide gated output signals indicative of the direction of the radar return which produces the gating signal. An inverse function generator 82 is responsively connected to the system trigger of the radar system 44 to provide signals to summing amplifier 81 which vary inversely with elapsed time. Since the time of occurrence of the signal return or gating signal from the radar system 44 subsequent to the occurrence of the system trigger is indicative of the range or distance of the target causing such return, the gating of the output of function generator 82 by gated means 81 produces a gated signal component indicative 1 of -, the inverse range of the target or terrain. R A potentiometer 83 attenuates the output of generator 82 by a fixed amount indicative of a desired perpendicular clearance distance H 0 , to provide a signal level indicative of the ratio Ho/R. Such signal represents the small angle approximation of a vertical clearance angle to be maintained relative to the sensed terrain profile in order